With the advent of extensive computer networks, such as the Internet, and the increased concern for maintaining certain information confidential, various methods have been developed for the secure transmission of information. Cryptography is the science of information security and includes the study of methods to “disguise” information so that only an intended recipient can read it. For example, various encryption algorithms have been developed to encrypt a message that is in a form known as plaintext, using an encryption key, to create what is known as ciphertext, transmit the ciphertext, and allow only persons possessing an appropriate decryption key to decrypt the ciphertext to obtain the plaintext.
A symmetric encryption algorithm is one wherein the encryption key and the decryption key are the same key. In order to maintain security, this key should be made known only to the sender and the recipient of the message and kept secret.
Other methods for secure messaging have also been developed, such as public key cryptography, which is an asymmetric key scheme that uses a pair of keys including a public key and a private key. In a typical process, a public key encrypts the plaintext to create the ciphertext, and a private key decrypts the ciphertext to obtain the plaintext. In such a case, the public key can be made known to anyone, while the private key is known by a single party and kept secret. In the typical process, the recipient of the message possesses the private key. The public/private key pair is generated such that, in general, it is computationally infeasible for someone to deduce the private key from knowledge of the corresponding public key. Thus, anyone who has a public key can encrypt information using that key, but without the corresponding private key, cannot decrypt it. Only the person who has the corresponding private key can decrypt the information. Examples of well-known algorithms using public key cryptography include Diffie-Hellman, and RSA (both named for their inventors). In general, the larger the key size, the more secure the algorithm becomes. In a typical cryptography system implementing an asymmetric key scheme, keys of at least 1024 bits are used, making such systems computationally intensive.
Another important aspect of secure communication is ensuring that certain information originates from a specific entity and has not been modified. One way to accomplish this using an asymmetric key scheme is with the use of digital signatures. In particular, a digital signature enables the recipient of information to verify the authenticity of the information's origin and can also be used to assure the recipient that the information is intact. Instead of encrypting information with a public key, information is encrypted with the sender's private key. If the encrypted information can then be decrypted with a public key corresponding to the sender's private key, that proves the information originated from one with possession of the private key, i.e., presumably the sender. Again, the security of this scheme depends on keeping the private key secret. Additionally, it depends on reliably linking a public key to the party having possession of the corresponding private key, in this case, the sender of the digital signature. The reliability of the linkage can be enhanced with the use of certificates, as more fully described below.
In order to for a recipient to insure that the received information has not been modified, one-way hash functions are used. A one-way hash function mathematically operates over a data block (of any size) to produce a specific output. If the data in the data block is changed at all, even by a single bit, a different result would be produced.
PGP (Pretty Good Privacy) is a known cryptography system that incorporates a symmetric key algorithm, an asymmetric key algorithm, and the use of digital signatures. In particular, when a user wants to send a message to an intended recipient, a session key is created, which is a one-time-only, randomly generated secret symmetric key that is used to encrypt the message body (plaintext) by a selected algorithm. The use of a symmetric session key on the message body is generally simpler and faster than using a public key, due to the size of the public/private key pairs required and the computationally intensive nature of the asymmetric key algorithms. The session key itself is then encrypted using the public key corresponding to the private key of the intended recipient of the message. This public key encrypted session key is transmitted along with the encrypted message body (ciphertext) to the recipient. The recipient then uses its private key to determine the secret session key, and uses the session key to decrypt the encrypted message body to obtain the message body.
With respect to authentication and integrity of the message, a PGP system uses a hash function on the message body (or a portion thereof) to compute what is known as a message digest or hash. Then, the sender uses the sender's own private key to encrypt the message digest to create a digital signature to be sent with the encrypted message body. As described above, the recipient is able to decrypt the encrypted message body. The recipient then re-computes the message digest by using the hash function on the message body. The digital signature is decrypted using the public key corresponding to the sender's private key to yield the message digest computed by the sender. If the message digest computed by the sender is the same as the message digest recomputed by the recipient, this provides the assurance that the message is from the sender (authentication), and the message body has not been modified (integrity).
A typical application for PGP is to enable secure e-mail communication. Each party has possession of its own secret private key, with an individual public key being associated with each private key. Two-way secure communication is possible between any two parties since a private key is assigned exclusively to a single party, while the corresponding public key is made known to any party.
A public key infrastructure (PKI) is an infrastructure that allows for the secure generation and distribution of public-private key pairs for use in asymmetric cryptography systems. In particular, a PKI provides for the binding of public keys to corresponding parties with the use of certificates, wherein each certificate presents a public key of one party to one or more other parties. A certificate authority (CA) that issues the certificate digitally signs it. A certificate authority essentially checks to ensure that a specified public key does indeed belong to a specified party, and attests to that fact with a digitally signed certificate. Various levels of certificate authorities are permitted, with a top level certificate authority able to establish lower level certificate authorities, which can also attest to public key ownership through the use of certificates.